Calculate Illuminance

Online calculator and formulas for lux calculations in lighting engineering

Illuminance calculator (JavaScript)

Fundamentals

Illuminance E in lux can be calculated from luminous flux Φ and area A or by the distance law E = I/r².

lm
m²
Result

Example calculations

Example 1: E from luminous flux and area

Given: Φ = 1200 lm, A = 12 m²

\[E=\frac{\Phi}{A}=\frac{1200}{12}=100\,lx\]

Result: E = 100 lx

Example 2: E from distance law

Given: I = 300 cd, r = 2 m

\[E=\frac{I}{r^2}=\frac{300}{2^2}=75\,lx\]

Result: E = 75 lx

Example 3: Impact of distance

Given: I = 300 cd

\[E(1m)=300\,lx,\] \[E(2m)=75\,lx,\;E(3m)=33.33\,lx\]

Interpretation: E decreases with the square of distance.

Formulas and comprehensive illuminance notes

Illuminance E describes how much luminous flux reaches a surface. It is a key photometric quantity for lighting design in offices, streets, homes, and industrial facilities. The unit is lux (lx), where 1 lx = 1 lm/m².

Area-based illuminance
\[E=\frac{\Phi}{A}\]
Distance law (point source)
\[E=\frac{I}{r^2}\]
Area from flux and illuminance
\[A=\frac{\Phi}{E}\]
Intensity from illuminance and distance
\[I=E\cdot r^2\]
Practical guidance
The distance law is idealized for point sources in free space and perpendicular incidence. In real rooms, reflections, beam pattern, luminaire optics, and incidence angle influence the actual illuminance. For compliant lighting design, maintenance factors, uniformity, and glare evaluation should also be considered.

Description

What is illuminance?

Illuminance E is a photometric quantity that describes the amount of luminous flux Φ striking an illuminated surface per unit area. It therefore indicates how brightly a surface is actually illuminated—regardless of how bright the light source itself appears. The SI unit is the lux (lx):

\[1\,lx = 1\,\frac{lm}{m^2}\]
Calculation formulas

Depending on the available quantities, two formulas are available:

  • Area formula: Luminous flux Φ (in lumens) on area A (in m²):
    \[E = \frac{\Phi}{A}\]
  • Inverse-square law: for a point light source with luminous intensity I (in candela) at a distance r (in meters):
    \[E = \frac{I}{r^2}\]
    Illuminance decreases with the square of the distance — doubling the distance results in only one-quarter of the illuminance.
Overview of photometric quantities
Quantity Symbol Unit Description
Luminous fluxΦlm (lumen)Total light power emitted by a source
Luminous intensityIcd (candela)Luminous flux per solid angle in a specific direction
IlluminanceElx (lux)Luminous flux per unit area on the illuminated surface
LuminanceLcd/m²Subjectively perceived brightness of a surface
Typical reference values
  • 0.001 lx — Starlight on a moonless night
  • 1 lx — Candlelight at a distance of 1 m
  • 100 – 300 lx — Recommended lighting for offices and workplaces
  • 500 lx — Standard requirement for precision assembly work
  • 10,000 – 100,000 lx — Outdoor daylight
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